Operation

Encipher the message with a standard 6x6 Polybius square (see that node for an example, but in short: A-Z and 0-9, in six lines horizontally) with the rows and columns labelled from 0 to 5.

Calculate the square root of the key and, beginning at the first digit after the decimal place, copy each digit which is within the range 0-5 until you have the same number of digits as in your message.

Add (modulo 6) the message and key streams. This is the ciphertext to be sent to your partner in crime.